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Update on Bollworm Situation in Soybean
Author: Gus Lorenz, Extension Entomologist

Bollworm numbers continue to run high in soybeans. We observed a field on Friday that was running 91 larvae per 25 sweeps just east of Waldenburg. The threshold is 9 per 25 sweeps.

Bollworm Trap

Bollworm trap shows evidence of situation

I have received calls and seen many fields running in excess of one per sweep and others in the two per sweep range. Based on our trap counts, we expect numbers to remain high through at least the middle of August. Treatment levels have been observed from the Bootheel of Missouri to the Louisiana line and up into the Arkansas River Valley. Fields at the most risk continue to be those that are blooming (or just before bloom) and setting pods and have not achieved canopy closure. In some cases, fields that are lapped are also experiencing bollworm problems.

With the high numbers of larvae and high heat, it’s not uncommon to see some slippage in control; however, in several cases, it also appears that we are also seeing some slippage with pyrethroids. Scott Akin reported on this last week, and the situation continues to be a problem. While budworms may be part of the problem, it appears to be bigger than just budworms.   Several fields where less than adequate control was achieved resulted in populations of bollworms that made it through the treatment. Listed below are our recommendations for control based on the current situation.

Suggestions for Bollworm Control in Soybeans:

  1. To insure adequate application, we recommend a minimum of 5 gallons per acre (GPA) by air and 10 GPA by ground. Also, consider the addition of an adjuvant, such as crop oil, with your insecticide application.
  2. For high populations, (2X threshold or greater) if using a pyrethroid, consider adding 0.5 lb/ A of acephate. Tankmixing the pyrethroid and acephate should improve both residual and percent control.  Maintain a higher-end rate with the pyrethroid; in other words don’t cut the rate.
  3. Consider other insecticides. We have observed excellent control with both Belt at 2 oz/ acre and Steward at 1 gal/ 20 acres. The advantages to these products are that they both provide good control of caterpillars in soybeans, and they have little impact on beneficial insects. However, for the same reason they don’t impact beneficial insects, they also provide no control for stink bugs and other non-caterpillar pests of soybean. Based on our observations these products have provided the most consistent control of bollworms.
  4. Scout fields closely following application to make sure adequate control was achieved and for subsequent problems with bollworms and other pests.

There has been considerable discussion with Belt regarding rate because it is a new product in soybean. It is my opinion that the 2 oz. rate is adequate, and observations we have made indicate it is doing an excellent job.  It is rather expensive and the 3 oz. rate may provide more residual control, but most growers will be hesitant to spend the money. Remember, it’s easy to make a recommendation when it’s not your money.

Bollworm on soybean

Bollworm on soybean

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